1. Field of the Invention
The subject matter of the present invention is a method for quantitatively determining the airborne portion and solid-borne portion of engine noise such as can be heard, for example, in the interior of motor vehicles or electric vehicles. By flooding the engine compartment with a gas or gas mixture with a different density than air, the acoustic conditions in the engine compartment are changed. Conclusions about the airborne and solid-borne portions of the engine noise in the interior of the vehicle can be drawn from measurements taken before and after this acoustic change. The method can also be applied to other forms of transportation, such as ships and aircraft, as well as power stations, machine shops, etc.
2. Background of the Invention
To achieve a high level of driving comfort in motor vehicles, a pleasant noise level in the passenger compartment of a vehicle is desirable. This noise level or sound level is determined to a considerable degree by the drive unit, in one example, a reciprocating internal combustion engine. The noise emitted by the engine is usually transmitted into the passenger compartment in two ways: noise transmitted directly into the passenger compartment as airborne sound; and sound emitted by the engine transmitted into the passenger compartment through the engine block to the body of the vehicle as solid-borne sound. To selectively reduce the sound level, it is desirable to separate the portion of solid-borne sound from the portion of airborne sound.
The prior art, with respect to the reduction of noise levels, is directed toward reducing airborne sound level of the engine. Thus, for example a test bench for engine sound measurement from BMW AG is described in detail in ATZp. 446 et seq. (1997).
In the same issue of ATZ, p. 414 et seq., measures for reducing the level of airborne sound by sound damping and measures for reducing the level of solid-borne sound by adjustable hydraulic engine bearings are disclosed using the example of the Audi A8.
The solid-borne and airborne portions of the engine noise, as heard in the interior of the vehicle, can currently be determined only at great cost. Two basic methods are described in a paper xe2x80x9cGer xc3xa4 uschpfadanalyse einmal anders [A Different Approach to Analyzing Noise Paths]xe2x80x9d for the 3rd Vehicle Acoustics Conference xe2x80x9cGer xc3xa4 uschminderung in Kraftfahrzeugen [Reducing Noise in Motor Vehicles]xe2x80x9d of 10/11.3.1992 at the Haus der Technik e.V., Essen. In the first method, the drive train is partially or even completely decoupled from the vehicle so that the noise in the interior of the vehicle is only the airborne portion of the engine noise. However, for this purpose, costly steel frames must be set up for suspending the entire drive train. In particular, the decoupling of the drive shafts is problematic.
In addition, there is an indirect method in which preliminary measurements of the solid-borne and airborne sound transmission functions, known by the name xe2x80x9cTransferpfad-Analyse [Noise Path Analysis]xe2x80x9d, are determined. In addition, engine acceleration and noise emitted by the engine are measured. By superposing the signals, which are linear in large regions, it is possible to draw conclusions about the relative contributions of airborne and solid-borne sound. The precision of the conclusions is, however, frequency-dependent as the solid-borne sound transfer functions are imprecise, in particular at low and high frequencies. At low frequencies, it is not possible to carry out excitation sufficiently because the coherence worsens. At high frequencies, the phase can only be measured imprecisely. In addition, the transfer functions also change with temperature (temperature-dependent elasticity of the rubber bearings) and engine load (tensioning of the engine bearings, for example, full-load accelerations at low rotational speeds).
The precise measurement of the airborne sound emissions of the engine is also problematic. Such measurements are carried out in the acoustic close-range field of the engine, as a result of which the sound pressure between two adjacent points can vary greatly. It is, therefore, impossible in practical terms to pick up the emitted engine noise completely and correctly using measuring equipment. Typically, only one microphone is used on each side of the engine to record the engine noise.
Methods in which acoustic effects are studied using gases with properties other than those of as air are known. To influence the acoustics in aircraft, for example, xe2x80x9clightweightxe2x80x9d gases, i.e., gases with low density or a high speed of sound are used. In FR 2783498, it is proposed to cause lightweight gases to flow out at the rotor blades of a jet engine to minimize emission of airborne sound. FR2783499 discloses a method in which the shock waves occurring to the engine inlet and to the front edges of the aircraft airfoils when flying above the speed of sound are reduced by causing a lightweight gas to flow out in a selective way. GB 2271387 describes resonators integrated in an aircraft jet engine, which are filled with a lightweight gas, as a result of which the inherent frequencies of the resonator, and thus the effect of the resonator, are influenced in a selective way. In addition to these methods, which utilize the acoustic properties of low-density gases, methods for quantitatively determining gas properties with acoustic means are also known. Thus, DE 19745954 A1 describes a measuring method in which the composition of gases (for example, for anesthetic applications) is determined acoustically. Here, sound pressure oscillations are generated in the gas mixture to be measured and the propagation speed, the sound pressure amplitude, the frequency changes or other acoustic variables of the gas space are determined and the gas composition derived therefrom.
An advantage of the present invention is in providing a simple measuring method with which the airborne portion and solid-borne portion of engine noise can be determined easily in a qualitative and quantitative fashion for any operating point and any frequency.
This is achieved by a measuring method for determining the solid-borne portion and the airborne portion of powerplant noise in the interior of a vehicle, by performing a first sound measurement by at least one microphone in an engine compartment and at least one microphone in the interior of the vehicle, the microphones being connected to at least one amplifier and evaluation unit. A gas other than air is introduced into the engine compartment of the vehicle. A second sound measurement is performed when the gas, other than air is in the engine compartment. The first and second sound measurements are compared.
According to the invention, the entire engine compartment or parts thereof is flooded with a gas or gas mixture, which has a density or speed of sound, which is significantly different from that of air. In addition, this gas can also be introduced into the space below the engine cavity and under the floor of the vehicle. The gas used here is preferably helium, hydrogen, xenon or sulfahexafluoride, but other gases or gas mixtures can also be used. Helium, for example, has a higher speed of sound than air by a factor of three, and the speed of sound in xenon and sulfahexafluoride is on the other hand lower than that of air. By flooding the engine compartment with one of these gases, the acoustic conditions change, i.e., the modes shift to higher frequencies in helium and to lower frequencies in xenon and sulfahexafluoride. Here, the airborne sound emitted by the drive train is modified to a very great extent, which can also be picked up by measuring equipment in the interior of the vehicle. In the case of helium, for example, the sound pressure is as a rule lower at low frequencies. However, the sound pressure can also rise at medium and high frequencies.
If it is to be qualitatively decided whether the engine noise in the interior of the vehicle is also influenced by the airborne portion of the engine noise at a specific frequency, or is mainly made up of the solid-borne portion, at least one microphone is installed in the interior of the vehicle to measure the sound, and the sound signals are transmitted to an evaluation unit and analyzed there (this is assumed in the text which follows and is no longer mentioned separately). If significant changes in sound pressure are measured in the interior of the vehicle when there is a given load state and rotational speed and a specific frequency, or averaged or cumulated over all frequencies, after the gas or gas mixture has been introduced into the engine compartment, it is possible to infer therefrom that the airborne portion of the engine noise is predominant in the engine noise in the interior of the vehicle. If only slight changes, or no changes at all, can be detected in the sound pressure measured in the interior of the vehicle, it is possible to assume that the solid-borne sound induced in the vehicle by the engine is dominant.
For a more precise, quantitative analysis, the airborne sound in the engine compartment is additionally measured at one or more characteristic points and measured in the interior of the vehicle, using one or more microphones, before, during and after the flooding with gas. To control the phase and the engine speed, the rotational speed signal or the acceleration of the engine block can also be measured. The average rotational speed also can easily be determined from the sound pressure measured in the interior of the vehicle or in the engine compartment. The corresponding analytical techniques are known.
Through the rise and fall of the sound pressure levels in the engine compartment and in the interior of the vehicle after and during the flooding with gas it is possible to calculate the solid-borne sound portion and the airborne sound portion in the interior of the vehicle for the individual frequencies.
One possible way of calculation is discussed below. It is assumed that an internal combustion engine is run up in the declutched state when the vehicle is stationary (i.e., no rolling or wind noises). At a specific engine operating point and a selected frequency, the overall sound pressure level Lges, made up of the airborne sound portion LL and the solid-borne sound portion LK of the engine noise, is measured in the interior of the vehicle. Assuming uncorrelated sound pressure signals (i.e., there is no fixed phase relation between the sound in the engine compartment and in the interior of the vehicle), the overall sound pressure Lges in the interior of the vehicle follows from the summing of the energy of the sound pressures of the airborne sound portion and the solid-borne sound portion which is applied when there are uncorrelated sound pressure portions (cf. H. Klingenberg, Automobil-Me xcex2 technik [Automobile Measuring Technology], Vol. A: Akustik [Acoustics], Springer-Verlag 1988, p. 13). Here, the two individual levels of the airborne sound LL and of the solid-borne sound LK are summed as follows (i.e., here n=2):                               L          ges                =                  10          ⁢                      xe2x80x83                    ⁢                      lg            (                                          10                                                      L                    L                                    10                                            +                              10                                                      L                    K                                    10                                                      )                                              (        I        )            
In the engine compartment, the airborne sound pressure level LM is measured in the initial state. After the flooding with gas, a sound pressure level, which is lower by xcex94LM is measured in the engine compartment. It is to be ensured, beforehand, that the microphone is also appropriately standardized in the gas or gas mixture or in air (if appropriate, a special microphone is used). Assuming that there is a linear sound pressure transmission to the interior of the vehicle, the airborne sound portion of the engine noise in the interior of the vehicle also decreases by xcex94LL=xcex94LM, which is assumed in the following calculation. If the linearity of the airborne sound transmission were not to apply, a corresponding correction function is to be determined and is to be taken into account in the determination of xcex94LL from xcex94LM and the transmission function is to be determined in both states, i.e., before and after the flooding of the engine compartment with gas. Here, it is appropriate to carry out a reciprocal measurement (sound excitation in the interior of the vehicle and measurement of the sound signal in the engine compartment). In addition, it is to be assumed that the flooding with gas does not influence the emitted solid-borne sound of the engine or the solid-borne transmission into the interior of the vehicle, i.e., LK remains unchanged before and after the flooding with gas. The following formula describes the state in the interior of the vehicle after the flooding with gas:                                           L            ges                    -                      Δ            ⁢                          xe2x80x83                        ⁢                          L              ges                                      =                  10          ⁢                      xe2x80x83                    ⁢                      lg            (                                          10                                                                            L                      L                                        -                                          Δ                      L                                                        10                                            +                              10                                                      L                    K                                    10                                                      )                                              (        II        )            
The airborne sound pressure level of the engine noise in the vehicle compartment is obtained, after the conversion of the equation, as:                               L          L                =                  10          ⁢                      xe2x80x83                    ⁢                      lg            (                                          10                                                      L                    ges                                    10                                            ⁢                              xe2x80x83                            ⁢                                                1                  -                                      10                                                                                            -                          Δ                                                ⁢                                                  xe2x80x83                                                ⁢                                                  L                          ges                                                                    10                                                                                        1                  -                                      10                                                                                            -                          Δ                                                ⁢                                                  xe2x80x83                                                ⁢                                                  L                          L                                                                    10                                                                                            )                                              (        III        )            
and the solid-borne sound level of the engine noise in the interior of the vehicle is consequently:                               L          K                =                  10          ⁢                      xe2x80x83                    ⁢                      lg            (                                          10                                                      L                    ges                                    10                                            -                              10                                                      L                    L                                    10                                                      )                                              (        IV        )            
A short example for illustrative purposes: the overall sound pressure level in the interior of the vehicle is assumed to be 90 dB at a specific frequency Lges. After the flooding with gas, the sound pressure level in the engine compartment decreases at the abovementioned frequency by xcex94LL=xcex94LM=10 dB, while in the interior of the vehicle the sound pressure level decreases by 2 dB. With the abovementioned formulas, the airborne sound pressure level LL is consequently 86.1 dB and the solid-borne sound pressure level LK is 87.7 dB. The levels and the sound pressures or sound output can also be measured in relation to one another to obtain relative data. Analogous calculations can also be carried out for correlated signals taking into account the phases of the sound pressure in the engine compartment and in the interior of the vehicle. The precision of the measuring method can be increased further by using a plurality of microphones in the interior of the vehicle (for example, at the ear positions of the individual passengers on the front seats and rear seats) and/or engine compartment. Another possible way of increasing the precision of the measuring method is to measure the sound pressure level at a constant operating point, while the gas or gas mixture is introduced into the engine compartment, and to measure it at different gas concentrations, and then compare the measured values.
In another advantageous refinement, not only the engine compartment itself but also the space between the engine compartment and the road or else the entire space under the vehicle or around the vehicle is filled with gas or gas mixture.
In one refinement for measuring the solid-borne portion and the airborne portion of the noise in the interior of the vehicle, the engine compartment is either sealed off or packed in a gas-tight fashion with a tarpaulin, and the gas or gas mixture is then introduced into the resulting closed-off cavity.
In a further preferred refinement, the gas or gas mixture is introduced into the engine compartment together with a foam or as an aerosol. The foam serves here as a carrier for the gas so that the gas cannot escape from the engine compartment as easily. In addition, this measure increases the damping of the airborne sound.
In a further preferred refinement, the gas or gas mixture is introduced into bags composed of a thin film and placed in the engine compartment. The xe2x80x9cgas bagsxe2x80x9d may, for practical reasons, also be filled with the gas after they have been placed in the engine compartment, through valves provided for that purpose. In addition to the gas, additional absorbent materials (foam, fabric, foam balls, wool or the like) can also be placed in the gas bags to increase the damping of sound.
In a further advantageous refinement, the vehicle is placed in an enclosed space. This space may be a test chamber or else a gas-tight film tent, a gas-tight enclosure or a similar, closed-off volume. The sealing of the engine space, which is possibly necessary to prevent the loss of gases, is thus eliminated. It is also possible to suck out the gas or gas mixture again and re-use it after the termination of the tests.
A further advantageous refinement is obtained when additional cavity bulkheads are used. Here, free regions in the engine compartment are separated off, for example, by metal sheets, plastic panels, reverberating films or the like. Thus, additional tuning of the engine space modes is obtained.
A further refinement provides for absorbent materials to be used in the engine compartment. For this purpose, resonators, damping wedges, foam elements, damping foils or the like are introduced in the engine compartment. In this way, the airborne sound damping is also increased. By introducing the gas or gas mixture into the engine compartment, the irradiation of airborne sound into the exterior is also changed. In this way, it is possible in the manner described above to determine the engine airborne sound portion of the overall external noise for various load states, for example on a vehicle dynamometer test facility.
Furthermore, the invention provides a device which is suitable for carrying out the methods described above. This device may be, for example, a measuring space, a test chamber or the like.
The above advantages and other advantages, objects, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.